Removal of ultrafine colloidal particles from shale gas fracturing flowback fluid by microbubble-intensified flocculation: Effect of microbubble size

Shale gas fracturing flowback fluid (FFF) is characterized as the hazardous wastewater with high viscosity, large displacement, and complex components, which is difficult to deal with. Flocculation has been proven to be the most extensively applied treatment process considering its effortless operation, low energy consumption, and comparatively high efficiency. However, efficient removal of ultrafine colloidal particles (<10 μm) from FFF still remains a big challenge since their extremely stable properties. Herein, microbubbles (MBs) were introduced into the flocculation reaction to enhance the flocculation performance. When the median particle diameter (d50) of MBs reached 0.97 μm (which matched the size of selected colloidal particles), the turbidity removal rate was as high as 91.93 % at 0.70 g/L flocculant dosage, which was superior to traditional stir flocculation without MBs (66.67 %). Under this condition, the mean floc size reached the maximum value of 64.77 μm at 30 s after adding the flocculant. Meanwhile, the floc fractal dimension was significantly improved and the flocs held more resilient, which was beneficial for solid-liquid separation. The MBs worked by the trapping and bridging functions. Consequently, microbubble-intensified flocculation provided an advantageous way for the effective removal of ultrafine colloidal particles in FFF by controlling the bubble size appropriately.